Methane emissions from wetlands: biogeochemical, microbial, and modeling perspectives from local to global scales

Abstract

Understanding the dynamics of methane (CH4 ) emissions is of paramount importance because CH4 has 25 times the global warming potential of carbon dioxide (CO2 ) and is currently the second most important anthropogenic greenhouse gas. Wetlands are the single largest natural CH4 source with median emissions from published studies of 164 Tg yr(-1) , which is about a third of total global emissions. We provide a perspective on important new frontiers in obtaining a better understanding of CH4 dynamics in natural systems, with a focus on wetlands. One of the most exciting recent developments in this field is the attempt to integrate the different methodologies and spatial scales of biogeochemistry, molecular microbiology, and modeling, and thus this is a major focus of this review. Our specific objectives are to provide an up-to-date synthesis of estimates of global CH4 emissions from wetlands and other freshwater aquatic ecosystems, briefly summarize major biogeophysical controls over CH4 emissions from wetlands, suggest new frontiers in CH4 biogeochemistry, examine relationships between methanogen community structure and CH4 dynamics in situ, and to review the current generation of CH4 models. We highlight throughout some of the most pressing issues concerning global change and feedbacks on CH4 emissions from natural ecosystems. Major uncertainties in estimating current and future CH4 emissions from natural ecosystems include the following: (i) A number of important controls over CH4 production, consumption, and transport have not been, or are inadequately, incorporated into existing CH4 biogeochemistry models. (ii) Significant errors in regional and global emission estimates are derived from large spatial-scale extrapolations from highly heterogeneous and often poorly mapped wetland complexes. (iii) The limited number of observations of CH4 fluxes and their associated environmental variables loosely constrains the parameterization of process-based biogeochemistry models.

Methanogenic food web in the gut contents of methane-emitting earthwormEudrilus eugeniaefrom Brazil

“…This generalization for the earthworm gut is in contrast to other invertebrates whose guts are heavily colonized by novel endemic microbes (for example, termites; Brune, 2006;Dietrich et al, 2014). The methanogenic food webs of classic methane-emitting ecosystems such as ruminants and wetlands are primarily linked to the initial hydrolysis of autotrophically produced polymers (for example, the hydrolysis of plant-synthesized lignocelluloses; Morrison et al, 2009;Bridgham et al, 2013). In contrast, the methanogenic food web of E. eugeniae is likely linked to the hydrolysis of heterotrophically produced polymers, that is, the hydrolysis of earthworm-derived polysacchariderich mucus (Wü st et al, 2009a).…”

The global methane budget 2000–2012

“…Most of this methane is oxidized in the underlying soil, while some is dissolved in the floodwater and leached away. The remaining methane is released to the atmosphere, primarily by diffusive transport through the rice plants, but also methane escapes from the soil via diffusion and bubbling through floodwaters (USEPA, 2016;Bridgham et al, 2013).…”

Plant-mediated methane and nitrous oxide fluxes from a carex meadow in Poyang Lake during drawdown periods

“…Global wetlands were estimated to account for a third of the current annual CH 4 emissions, and they are the dominant natural source of atmospheric CH 4 fluxes and their interannual variability (Bridgham et al 2013;IPCC 2013). Meanwhile, wetlands are recognized as considerable N 2 O emitters with an annual rate of 0.36 kg N 2 O-N ha −1 yr −1 (Zhuang et al 2012).…”

“…CH 4 and N 2 O emissions from wetlands are subjected to a number of biotic and abiotic controls Bridgham et al 2013), among which plants are considered to be the major one because they can strongly affect CH 4 and N 2 O production, consumption, and transportation (Rückauf et al 2004;Laanbroek 2010). Plant presence can increase soil CH 4 flux by enhancing substrate availability.…”